Defining Good Tone
What is “good” tone? It seems like a subjective inquiry that will lead to unjustifiable arguments; however, I believe that today’s classical guitarists can easily come to an agreement on whether a tone is either harsh or pleasant.
The modern classical guitar and its technique are the product of centuries of development. The scale length establishes a particular range that the cavity and plate resonances are intended to amplify in the most efficient way to its audience. Those variables narrow the field of success results.
When we speak or sing we change the shape of our oral and nasal cavities differently to form each vowel sound. Try singing, A, E, I, O, U. Within the context of an American English dialect you might notice that singing or speaking “A” is the vowel that is formed with the least amount oral or nasal manipulations. In fact, it is so comfortable that we use variations of this vowel sound to fill the silence when we are trying to form our actual words. “E” and “I” are typically more harsh and require use of the nasal cavity and a more open mouth, while “O” and “U” demand that we enlarge our oral cavity and close our mouths more. Thus, “A” or singing “Ah” should be the standard vowel sound for the guitar. Reference: http://www.indiana.edu/~emusic/acoustics/resonance.htm
Of course there are instruments of the orchestra that sound bright, nasal or tubby. They exist for that very reason. It’s their persona - they are designed for an intended effect.